In conventional Computer Assisted Information or Instruction (CAI) Systems, use is often made of a light pen. This pen is a tube containing a photodiode. When the tube is placed against a conventional raster scan TV picture tube and the scanning beam passes through its field of view, the resistance of the photodiode, and in some implementations a phototransistor, is sensed by the logic of the machine. The time when this occurs is correlated to the scan line number and horizontal position and the position of the light pen on the screen is derived. From this information, the logic assembly or computer takes appropriate action as, for example, to indicate a correct or incorrect answer to a question, or to cause the retrieval and reproduction of remedial instructions, or of stored information, or other control input by the student or operator.
In order to accomplish this effect in a conventional video-display system, it is necessary to count the number of scan lines to the line where the pen responds. It is further necessary to determine the amount of time required for the writing beam to accomplish a horizontal scan to the pen position.
A second method commonly used requires the logic portion of a computer to generate a lighted "area" which may be positioned at various locations on the screen or may be used to scan the full face of the display device in a manner similar to raster scanning. This is a coarser version of the approach described above.
Both these approaches generate the scanning image electronically and create a need for complex electronic circuitry to determine the timing and then to convert this information to a message defining the pen position in a form that is compatible with digital systems. These approaches also require the user to be knowledgeable in computer programming in order to use such systems, and further requires the additional complexity of a computer in order to make use of the pen position information that is presented as a digital signal to the digital computer by the complex electronic circuitry. The costs associated with such devices run into the hundreds of dollars and create an additional requirement in that an extensive computer logic equipment and an interfacing routine is required to convert the digitized pen position data to appropriate response by the operating program in a CAI or information retrieval system.